In parallel with the requirements on the performance and efficiency factor of gas turbine sets, there are increased requirements on the cooling of the machine components subject to high thermal loads on the one hand, and on the design of the cooling system on the other hand. Thus sufficient cooling has to be ensured in the interests of operating safety. On the other hand, the cooling air consumption has to be limited as far as possible. It was proposed in EP 62932 to cool the components of a gas turbine with steam in a closed circuit. This requires a comparatively expensive sealing of the components conducting the cooling steam. A purely convective cooling of the components takes place at the same time; the effect of a cooling film for reducing heat entry is hereby dispensed with. In a number of further documents, such as EP 684 369 or EP 995 891 and U.S. Pat. No. 6,161,385 corresponding thereto, it is proposed to use steam or a steam-air mixture for the cooling of film-cooled components. However, such methods consume comparatively large amounts of steam, which has to fulfill high requirements on purity and superheating so that blockage of the film cooling bores, often only a few tenths of a millimeter wide, does not occur. Even if the required steam quantity and quality are available, cooling of the gas turbine set with steam, instead of with compressor bleed air, is not inherently more reliable.
Consequently, cooling with compressor bleed air has as usual a series of well-founded advantages, the amount of cooling air withdrawn being minimized in the interest of the working process. Consequently, the cooling air system is designed always closer to the limits, in order to ensure sufficient cooling in the—from the cooling technology viewpoint—unfavorable operating point, while not using more cooling air than absolutely necessary. This means, on the one hand, a high sensitivity to deviations of the working process from the design point of the cooling, if, for example, the amounts of cooling air vary due to displacements of the pressure ratios in a machine. On the other hand, an overcooling of the thermally stressed components results in a series of other operating points, so that the performance and efficiency factor potentials remain unexploited.
It was therefore occasionally proposed, for example, in EP 1 028 230, to arrange variable throttle points in the cooling air path. DE 199 07 907 proposes direct adjustment of the initial pressure of the cooling air by means of adjustable compressor blade rows which are arranged immediately neighboring a bleed point for cooling air. Although the implementation of this measure is promising, it is, of course, very expensive, and scarcely suited just for a retro-fitting of existing gas turbine sets. Besides, the building of movable parts into the cooling air system holds the latent danger of blockage of the cooling air ducts on failure of mechanical components.
A further relevant question is the feed of cooling air to structures in the region of the combustor or to the front side of the first guide blade row of a turbine. While it is sought to minimize the pressure loss of the working medium, and thus to keep the pressure at the turbine inlet as close as possible to the compressor end pressure, a sufficient cooling air mass flow has to pass through narrow cooling air channels and cooling bores. This of course requires a corresponding pressure drop over the cooling air system, so that the initial pressure of the cooling air system also cannot be higher than the compressor end pressure. Thus also in this regard only an appropriate, but not finally completely satisfactory, compromise can be found between the performance and efficiency factor data of a gas turbine set, on the one hand, and ensuring that there is sufficient cooling.